Refactor configuration object to class based (#4298)

* Create base class

* Use new interface for build.formats

* Implement config.formats

* Linter

* Test build.python

* Implement build.python property

* Test buil.config property

* Implement config.build property

* Test for base and name properties

* Implement missing properties

* Implement all abstract properties

* Remove setter

* Move some logic from Wrapper object

* Thinner Wrapper object

* Linter

* Remove output_base from dictionary

* Use build_image property

* Use wrapper attribute

* Use type attribute

* Use base attribute

* Use python property

* Update docs

* Use python property in wrapper

* Extra guard

* Linter

* Make config private

* Use formats attribute

* Use conda property

* Use requirements_file attribute

* Use conf_file attribute

* Introducing defaults

* Thinner wrapper

* Don't use ConfigWrapper directly

* Use defaults in formats

* Use load_yaml_config inside PythonEnvironment

* Remove dict inheritance

* Test for ConfigNotSupportedError

* Raise ConfigNotSupportedError when accessing a invalid attribute

* Test for validate

* Move version

* Docs

* More tests

* Remove try except

* More tests

* Linter

* Move _config to base class

* Implement default for use_system_site_packages

* Typo

* Move build_image from ConfigWrapper

* Move requirements_file from ConfigWrapper

* Move python_version from ConfigWrapper

* Move python_full_version and python_interpreter from ConfigWrapper

* Pass build_image as default

* Remove ConfigWrapper from tests

* Goodbye ConfigWrapper

* Fix tests

* More tests

* Rename module

* Fix conflict

* Trailing comma

* Add TODOs

* Rename exception

* Update copy

Author: stsewd